feat: client-selectable compositor (protocol → host → client → C ABI → mgmt → web)

A client can now request which compositor backend the host drives its virtual
output on (gamescope/KWin/Mutter/wlroots). The host honors the request if that
backend is available, else falls back to auto-detect and reports the resolved
choice back — wire-compatible both directions (no ABI bump).

Protocol (punktfunk-core):
- New CompositorPref (config.rs): Auto|Kwin|Wlroots|Mutter|Gamescope with
  u8/name mappings. Appended as one optional byte to Hello (client preference)
  and Welcome (host's resolved choice). Both decoders already tolerate trailing
  bytes, so old↔new interop is preserved — ABI_VERSION stays 2. Round-trip +
  back-compat (truncated-message) tests.
- C ABI: punktfunk_connect_ex(compositor) + PUNKTFUNK_COMPOSITOR_* constants;
  punktfunk_connect delegates with AUTO, so the existing symbol is unchanged.
  NativeClient::connect / worker_main thread the preference through.

Host:
- vdisplay::available() enumerates usable backends via cheap, side-effect-free
  probes (KWin zkde global, gamescope binary+version, GNOME/Sway env), plus
  Compositor id/label/as_pref/from_pref/all helpers.
- m3 handshake resolves the preference to a concrete backend during the
  handshake (pick_compositor pure + resolved logging), reports it in Welcome,
  and threads it into virtual_stream (replacing the unconditional detect()).
- mgmt GET /v1/compositors lists every backend with availability + the
  auto-detected default (OpenAPI regenerated).

Client:
- punktfunk-client-rs --compositor NAME; logs the host's resolved choice from
  the Welcome ("session offer … compositor=…").

Web console:
- Host page gains a Compositors card (availability + default badges) via the
  codegen'd useListCompositors hook; en/de strings added.

Also fixes a pre-existing, env-dependent test-isolation bug:
mgmt::tests::paired_clients_list_and_unpair seeded the real
~/.config/punktfunk/paired.json (AppState::new loads it), so a real
GameStream-paired client leaked into body[0] on a dev box — now cleared first.

Live-validated against headless KWin: --compositor kwin honored, --compositor
mutter falls back to kwin (available=[kwin, gamescope]), resolved choice
round-trips to the client. Tests: +6 (wire/back-compat, resolution precedence,
endpoint); workspace green, clippy/fmt clean, C ABI harness PASS at abi_version=2,
web typecheck + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

include/punktfunk_core.h

@@ -19,6 +19,24 @@
 // added `punktfunk_pair` / `punktfunk_generate_identity` / `punktfunk_connection_request_mode`.
 #define ABI_VERSION 2
 
+// Compositor preference for [`punktfunk_connect_ex`] (`compositor` arg). `AUTO` lets the host
+// pick (auto-detect from its running desktop); a concrete value is honored only if that backend
+// is available on the host right now, else the host falls back to auto-detect. The resolved
+// choice is reported back over the protocol (see `punktfunk/1` `Welcome`).
+#define PUNKTFUNK_COMPOSITOR_AUTO 0
+
+// KWin / KDE Plasma.
+#define PUNKTFUNK_COMPOSITOR_KWIN 1
+
+// wlroots (Sway / Hyprland).
+#define PUNKTFUNK_COMPOSITOR_WLROOTS 2
+
+// Mutter / GNOME.
+#define PUNKTFUNK_COMPOSITOR_MUTTER 3
+
+// gamescope (spawned nested).
+#define PUNKTFUNK_COMPOSITOR_GAMESCOPE 4
+
 // 16-byte AEAD authentication tag appended by GCM.
 #define TAG_LEN 16
 
@@ -350,7 +368,8 @@ PunktfunkStatus punktfunk_get_stats(PunktfunkSession *s, PunktfunkStats *out);
 
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Connect to a `punktfunk/1` host and start a session at `width`x`height`@`refresh_hz`.
-// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure.
+// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure. Equivalent to
+// [`punktfunk_connect_ex`] with `compositor = PUNKTFUNK_COMPOSITOR_AUTO`.
 //
 // Trust: `pin_sha256` (NULL or 32 bytes) is the expected SHA-256 fingerprint of the host's
 // certificate — a mismatching host is rejected. NULL = trust on first use; persist the
@@ -378,6 +397,27 @@ PunktfunkConnection *punktfunk_connect(const char *host,
                                        uint32_t timeout_ms);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Like [`punktfunk_connect`], but requests a specific `compositor` backend on the host (one of
+// the `PUNKTFUNK_COMPOSITOR_*` values). `PUNKTFUNK_COMPOSITOR_AUTO` (or any unrecognized value)
+// lets the host decide; a concrete value is honored only if available, else the host falls back
+// to auto-detect. The resolved choice is logged host-side and returned over the protocol.
+//
+// # Safety
+// Same as [`punktfunk_connect`].
+PunktfunkConnection *punktfunk_connect_ex(const char *host,
+                                          uint16_t port,
+                                          uint32_t width,
+                                          uint32_t height,
+                                          uint32_t refresh_hz,
+                                          uint32_t compositor,
+                                          const uint8_t *pin_sha256,
+                                          uint8_t *observed_sha256_out,
+                                          const char *client_cert_pem,
+                                          const char *client_key_pem,
+                                          uint32_t timeout_ms);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Generate a persistent client identity: a self-signed certificate + private key, both
 // PEM, NUL-terminated, written into the caller's buffers. Generate ONCE, store both